Hydraulic fracturing has been heretofore conducted in mining of crude oil, natural gas and the like, and in recent years, utilization of hydraulic fracturing has spread with advancement of techniques for mining shale gas and shale oil.
In hydraulic fracturing, a high pressure is applied to the inside of a well to form a fracture (crack) in a mining layer, a support material such as sand is introduced into the fracture to prevent the fracture from being closed, and a passage having high permeability to gas or oil is provided in the mining layer to mine gas or oil. In application of a high pressure to the inside of a well, generally a high-viscosity fracturing fluid with a support material (e.g. sand), a gelling agent and so on contained in water is injected under pressure.
Such a fracturing fluid is required to have a viscosity which ensures that a sufficient fracture can be formed in a mining layer and a support material such as sand can be carried to the fracture. Further, the fracturing fluid is recovered from the inside of the well after formation of the fracture, and is therefore required to have a reduced viscosity during recovery. Therefore, the fracturing fluid is desired to be designed in such a manner that it has a sufficiently high viscosity during fracture formation operation, and has a reduced viscosity during fracturing fluid recovery operation, and is thus easily recovered.
For example, Patent Document 1 discloses an aqueous composition to be used as a fracturing fluid, the aqueous composition containing (1) a hydrating polymer, (2) a peroxo compound substantially capable of generating free radicals in an amount sufficient to reduce the viscosity of an aqueous medium, and (3) a nitrite ion source. In Patent Document 1, a technique is proposed in which free radicals generated by the peroxo compound that reduces the viscosity of the aqueous composition are trapped by the nitrite ion source to suppress an early reduction in viscosity of the fracturing fluid which is caused by the peroxo compound.